1. Field of the Invention
The present invention concerns a communication system comprising a set of non-geostationary satellites providing communication links between ground stations and terminals, a communication link originating from a particular station and passing through a satellite terminating at associated terminals located within a limited geographical area including that station and providing a communication channel for at least some of said associated terminals, this communication link being subsequently transferred to another satellite in a procedure referred to herein as communication link handover.
2. Description of the Prior Art
A system of this type is described in the article "GLOBALSTAR: a transparent system" by D. Rouffet, published in the journal Revue des Telecommunications, 1st quarter of 1993, and in the documents cited in the bibliography of the article.
In the system described in the above document the terminals are mobile rather than fixed. They have an omnidirectional antenna and communicate with the satellite(s) providing them with the strongest signal. Each satellite covers a plurality of areas on the ground by means of a plurality of beams with different carrier frequencies, and these areas (called "footprints") moves as the satellite moves. As seen from the ground, the movement of the footprints means that the stations and the terminals must frequently change frequency, which compromises the overall efficiency of communication. Moreover, the bandwidth that can be transmitted between a station and an associated terminal in this system is relatively small.
The present invention concerns a system in which the number of frequency changes is considerably reduced and which provides broadband links between stations and terminals.
Copending application Ser. No. 08/574309 filed Dec. 18, 1995 describes one communication system meeting these objectives in which the link between a particular station of this kind and the associated terminals is set up by a satellite from said set of satellites which is commanded to set up to this end a steerable transmit beam and a steerable receive beam pointing to one such limited area including said particular station and supporting said communication link.
The use of steerable beams that do not move relative to the ground enables one satellite to set up and to maintain a communication link between a station and its terminals while moving from one horizon as seen from the station, to the other horizon, i.e. for as long as possible, given that the satellites are non-geostationary, which contributes to reducing switching between links and therefore to improving the overall transmission efficiency.
By a process that is conventional in other applications, not only the stations but also the terminals communicate through the intermediary of the satellites using steerable antennas, which requires at least two antennas per station.
The invention is directed to enabling the use in a system of this kind of terminals having only one antenna or, more generally, terminals such that communication is interrupted at the time of link handover.